1. Field of the Invention
The invention provides an apparatus and a process for producing a single crystal of silicon, in which granular silicon is used as the raw material, and is processed to produce the single crystal. The apparatus comprises a plate having a central opening and a tube directed downward from the central opening, a device for metering the granular silicon onto the plate, a first induction heating coil arranged above the plate and a second induction heating coil arranged beneath the plate.
2. Description of the Related Art
U.S. 2003145781 A1 and U.S. 2011185963 A1 describe apparatuses having the features mentioned, and also processes for producing a single crystal using such apparatuses. The lower end of the downwardly directed tube is closed with solid silicon at first, and the tube is heated with the aid of the second induction heating coil, in order that liquid silicon forms at the lower end of the closed tube. Subsequently, a monocrystalline seed crystal is contacted with the liquid silicon, rotated and lowered and, at the same time, further silicon is melted from the lower end of the tube. When the seed crystal is lowered, liquid silicon crystallizes on the seed crystal. After a phase which is called necking, which serves the purpose of passing dislocations out of the crystallized part to the surface thereof, a growing single crystal is finally present. At the top end of the growing crystal, there is a small volume of a melt of silicon which supports the further growth of the single crystal. The volume of the melt and the diameter of the growing single crystal are increased constantly during a further phase until a target diameter has been attained. Thereafter, the single crystal is allowed to grow in an approximately cylindrical manner, in order to further process the part which has grown cylindrically to give semiconductor wafers at a later stage. After the solid silicon which closed the lower end of the tube has melted, further liquid silicon which is required for growth of the single crystal is provided. For this purpose, granular silicon is deposited on the plate and melted with the aid of the first induction heating coil, while rotating the plate. The silicon flows as liquid silicon from the top side of the plate to the central opening in the plate and through the tube to the melt atop the growing single crystal.
U.S. 2003145781 A1 proposes using a vessel made of quartz or a plate made of silicon as the plate, providing elevations on the top side of the plate which form passages in order to extend the flow path for the melting silicon to the central opening. The extension of the flow path is supposed to promote complete melting of solid silicon originating from the granular silicon before it arrives at the growing single crystal. If there are to be no such passages, it is proposed that a barrier of solid silicon be set up on the top side of the plate in the region of the central opening by the action of the first induction heating coil. The height of the barrier is chosen such that granular silicon cannot get over the barrier unless it has melted completely.
The inventors of the present invention have found that the top side of a plate made of silicon is altered by the action of the first induction heating coil to such an extent that the use of the plate for production of a further single crystal is not an option, or is possible only after extensive reconditioning. In addition, it was found that when a plate made of quartz is used, the transport of liquid silicon from the region of the outer edge of the top side to the inner edge of the top side is impaired even when there are no passages to extend the flow path.